In recent years, ever-higher priority is being accorded to developing state-of-the-art security-screening methods and devices that will rapidly and reliably detect small quantities (tens of grams) of illicitly-transported SNM. Sensitivity of this order is required to interdict their use in, for example, panic-creating radiological dispersion devices (RDD's) deployed in densely-populated areas, or SNM accretion for the fabrication of improvised nuclear devices (IND's). Indeed, effective countering of these threats is currently considered one of the highest-priority medium-term R&D challenges at the U.S. Dept. of Homeland Security (DHS), among other national and international law-enforcement agencies. For obvious reasons, the severity of the detection problem tends to increase sharply with the mass and volume of the objects to be screened.
The following are examples of the prior art:
a) Passive Radiation-Monitoring Systems:
Such “Portal Monitors” were developed for the purpose of interdicting theft or diversion of SNM from high-security sites. The detection of SNM is based on measuring an increase in radiation intensity above the ambient background. Among others, LANL (Los Alamos National Laboratories) and LLNL (Lawrence Livermore National Laboratories) have been active in developing such systems, which are usually packaged in three distinct categories: 1) Small, Hand-Held Monitors 2) Automatic Pedestrian Monitors 3) Automatic Vehicle Monitors.
Vehicle monitoring systems for detecting SNM are now available from several commercial companies. Among these, Canberra's JPM-12A Vehicle Monitor was developed jointly with LANL. It consists of two large plastic scintillators positioned on either side of the portal, that measure the gamma-rays emitted from SNM. The instrument can detect 10 gr of Pu and 1000 gr of HEU in an unshielded vehicle moving at a speed of 2 m/s. The disadvantage of this method is that gamma detection can be thwarted with relative ease by enveloping the SNM with Pb-sheet, since the minimum detected quantities increase rapidly by an order of magnitude when shielded by several mm of it. In the Pu case, the problem can be countered by using neutron detectors in the portal. However, due to low neutron emission rates, this approach requires large-area neutron detectors. LANL has developed such a system, based on two large-area 3He detectors between which vehicles pass. An equivalent system is currently marketed by TSA-Systems-Ltd., Model NVM-245. NUCSAFE's Vehicle Monitor claims detection of 5 gr of gamma-shielded weapons-grade Pu within 10 s at 1 m distance. Also available are gamma-neutron Vehicle Portal Monitors manufactured by LAURUS-Systems-Inc. Model No. VM-250GN and by POLIMASTER, Model Series PM5000. For the latter, which incorporate sizeable plastic-scintillator detectors as well as large-area 3He proportional counters, detection of 4.3 gr 239Pu and 300 gr 235U at a scan speed of 10 km/hr is claimed. However, in analogy to γ-rays, small Pu quantities might evade detection if some neutron shielding is introduced.
b) Single-Energy Radiography Systems
This method yields information about the contents of a screened object. A number of systems have been developed in recent years by commercial companies. Nearly all of them use high energy bremsstrahlung radiation produced by high-power linacs, although SAIC have built a line of products (VACIS) around a radioactive 60Co source. In several systems, two views at 90° to each other are generated, to obtain more information. Commercial companies such as Smiths-Heimann and Aracor produce linac-based vehicle and marine container inspection systems. They operate at electron energies of ˜10 MeV and employ Cadmium-Tungstate radiation detectors. The spatial resolution permits the detection of 1 mm copper wire. The scan time is ˜3 minutes for a 20 m-long vehicle, but the visual inspection time of the resulting image typically takes 10-15 minutes. These systems penetrate ˜30 cm of steel and the radiation dose to the object is about 150-250 μGy. Their obvious drawback is that they do not automatically identify SNM and in general, the performance of such systems relies heavily on operator skill and judgment.
c) Dual-Energy Radiography (DER) Systems
DER is a well-established technique that has found numerous applications in medical imaging (in particular, in-vivo bone mineral densitometry), environmental studies, material assaying, NDT, NDE, as well as security inspection scenarios. It is based on comparing the transmission attenuation at two energies, a sensitive measure for the atomic number Z of absorbers in the line-of-sight from radiation source to detector.
DER scans are usually performed at energies where the photoelectric component of the transmission attenuation dominates. One particular variant exploits characteristic discontinuities in attenuation when the incident photon energy is varied around inner-atomic-shell (K,L,M, . . . , ) binding energies of a particular element. Typically, the latter vary with Z2. Locating such an energy-discontinuity (by comparing the photon flux transmission above and below it) is a sensitive indication of the presence of the element in question in the field-of-view. However, this variant of the method is unsuitable for inspecting massive cargo such as aviation or marine containers, since the relevant K-binding energies (the highest-energy discontinuities) for Z˜92 are at ˜110 keV, an energy too low to penetrate the inspected items.
In summary, the inadequacy of existing screening methods to effectively interdict the illicit-transport of SNM at the crucial outgoing and incoming control points underscores the need for novel inspection systems that will reliably detect the presence of small quantities of SNM in cargo items ranging in magnitude from small packages, through passenger baggage, palletized cargo and aviation containers, up to a fall-size marine container, loaded land-vehicle or railroad freight-car.